Create New Task in Workflow and populate custom field

Create New Task in Workflow and populate custom field

The goal is to add the ability to schedule a follow up task on the same screen our users are entering their completed task. 

I have created 2 custom fields on my task screen:

Follow Up Task 1 (pick list) and Follow Up Date 1

I then created a workflow that created a new task if:

Follow Up Task 1 field is not empty
Task Status is "Complete"
then create a new task where Subject = Follow Up Task 1 and Due Date = Follow Up Date 1 + 0 days

However, I have a custom field on my task screen called Sales Task (picklist).  I also need this new task to populate the 'Sales Task' field with the Follow Up Task 1 value.  There is no way to do it on the Create Task in workflow as custom fields are not an option.  I need to create a custom function but don't know the syntax. 

Any help would be appreciated.